To determine the parameters we want (stellar magnitudes and colors) from the
observations we have, we fit the data to a model.
The model should represent the circumstances of the observations as
realistically as possible; otherwise, effects present in the data but not in
the model will be distributed among the available adjustable parameters, such
as stellar magnitudes, extinction coefficients, and transformation parameters.
The result may well be a good fit to the data, with small residuals; but the
parameters will be biased -- that is, they will have significant systematic
errors.
Such errors are called reduction errors by Manfroid and
Sterken [16]